1. Field of the Invention
The present invention relates to telecommunications systems using nQAM modulated digital data channels and, in particular, the provision of control channels in such systems. The present invention has particular application to VDSL transmission systems, although it may be used within system employing nQAM modulation.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA.multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive
NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
Another modulation scheme that can be used with VDSL is nQAM.
In certain types of transmission system using nQAM, it may be desirable to use a control channel superimposed on a nQAM data channel. In such circumstances, it is very desirable to be able to separate control channel data from user data without using a higher level protocol layer. For applications in multi-carrier systems with variable bit-loading it is particularly desirable that the control channel data be separable from user data when the value of n, i.e. the bit-loading, or constellation identifier, are unknown.
Accordingly, it is an object of the present invention to provide a telecommunications transmission system employing nQAM data modulation in which a control channel can be superimposed on a user data channel in a manner which permits separation of the control channel from the data channel without the use of a high level protocol layer when the QAM constellation identifier, n, is unknown.
According to a first aspect of the present invention, there is provided a method of transmitting binary data using nQAM modulation onto a carrier wave, characterised in that said binary data comprises both user data and control data, so that a control channel is superimposed on a user data channel within the same nQAM constellation, and in that separation of said control data is facilitated by arranging control data points to have either unique phase, or unique amplitude, values which are not employed for user data points.
Said control data may have unique phase values which are not employed for user data points.
Said unique phase values may fall substantially on the boundaries between quadrants of a nQAM constellation and said user data points have phase values which fall within quadrants of the nQAM constellation.
Said nQAM constellation may be partitioned so that (log2n-1) bits are available for user data and 2 bits are available for control data.
Control data points may constitute a 4QAM.
Said control data points may be uniquely identified regardless of the value of n.
According to a second aspect of the present invention, there is provided a data transmission system in which binary data is transmitted using nQAM modulation onto a carrier wave, characterised in that said binary data comprises both user data and control data, so that a control channel is superimposed on a user data channel within the same nQAM constellation, and in that separation of said control data is facilitated by arranging control data points to have either unique phase, or unique amplitude, values which are not employed for user data points.
Said data transmission system may be a DMT transmission system.
Said data transmission system may be a DMT based VDSL system.
Said data transmission system may include at least two transceivers each of which includes a transmitter and a receiver.
According to a third aspect of the present invention there is provided a receiver for use in a data transmission system, as described above, characterised in that said receiver has means for decoding an nQAM modulated carrier having a control channel and a user data channel modulated thereon, and means for separating said control channel from said user data channel without recourse to a higher layer protocol.
According to a fourth aspect of the present invention, there is provided a transmitter for use in a data transmission system as described above, said transmitter having means for modulating data onto a carrier wave as an nQAM constellation, said data comprising control data and user data, said control data modulated onto said carrier as an mQAM constellation, where m greater than n, and said mQAM constellation comprising a set of constellation points with unique values of phase, or amplitude, which unique values are not employed for the modulation of user data.
m may be equal to 4.
Said mQAM constellation may comprise a set of constellation points with unique values of phase, which values are not employed for user data constellation points.
n may be equal to 6.
According to a fifth aspect of the present invention, there is provided a transceiver, characterised in that said transceiver includes a receiver as described above.
Said transceiver may include a transmitter as described above.